The effects of calcium channel blockade on proliferation and differentiation of cardiac progenitor cells

Cardiogenesis depends on a tightly regulated balance between proliferation and differentiation of cardiac progenitor cells (CPCs) and their cardiomyocyte descendants. While exposure of early mouse embryos to Ca²⁺ channel antagonists has been associated with abnormal cardiac morphogenesis, less is known about the consequences of Ca²⁺ channel blockade on proliferation and differentiation of CPCs at the cellular level. Here we showed that at embryonic day (E) 11.5, the murine ventricles express several L-type and T-type Ca²⁺ channel isoforms, and that the dihydropyridine Ca²⁺ channel antagonist, nifedipine, blunts isoproterenol induced increases in intracellular Ca²⁺. Nifedipine mediated Ca²⁺ channel blockade was associated with a reduction in cell cycle activity of E11.5 CPCs and impaired assembly of the cardiomyocyte contractile apparatus. Furthermore, in cell transplantation experiments, systemic administration of nifedipine to adult mice receiving transplanted E11.5 ventricular cells (containing CPCs and cardiomyocytes) was associated with smaller graft sizes compared to vehicle treated control animals. These data suggest that intracellular Ca²⁺ is a critical regulator of the balance between CPC proliferation and differentiation and demonstrate that interactions between pharmacological drugs and transplanted cells could have a significant impact on the effectiveness of cell based therapies for myocardial repair.     

Regulation of the cardiac L-type calcium channel in L6 cells by arginine-vasopressin

L-type Ca2+ channel activity was measured in L6 cells as nifedipine-sensitive barium (Ba2+; 5 mM) influx in a depolarizing salt solution containing 140 mM KCl. Addition of AVP (arginine-vasopressin) during Ba2+ uptake reduced the rate of Ba2+ influx by 60-100%; this was followed by a gradual restoration of the initial rate of Ba2+ uptake. Blockade of PKC (protein kinase C) by pretreatment with 10 muM bisindolylmaleimide did not affect the initial inhibition of Ba2+ influx, but completely abolished the recovery phase. The effect of AVP was half-maximal at 10 nM AVP and was blocked by the V1a receptor antagonist d-(CH2)(5)-Tyr(Me)-AVP. Activation of G(alphas) by isoprenaline or cholera toxin antagonized the actions of AVP on Ba2+ uptake. This protection persisted in the presence of the PKA (protein kinase A) inhibitor KT5720, and was not mimicked by agents that increase cAMP. Inhibition of Ba2+ influx was also elicited by ATP and ET (endothelin 1) with an order of effectiveness ET相似文献   

Mutations in voltage-gated L-type calcium channel: implications in cardiac arrhythmia

The voltage-gated L-type calcium channel (LTCC) is essential for multiple cellular processes. In the heart, calcium influx through LTCC plays an important role in cardiac electrical excitation. Mutations in LTCC genes, including CACNA1C, CACNA1D, CACNB2 and CACNA2D, will induce the dysfunctions of calcium channels, which result in the abnormal excitations of cardiomyocytes, and finally lead to cardiac arrhythmias. Nevertheless, the newly found mutations in LTCC and their functions are continuously being elucidated. This review summarizes recent findings on the mutations of LTCC, which are associated with long QT syndromes, Timothy syndromes, Brugada syndromes, short QT syndromes, and some other cardiac arrhythmias. Indeed, we describe the gain/loss-of-functions of these mutations in LTCC, which can give an explanation for the phenotypes of cardiac arrhythmias. Moreover, we present several challenges in the field at present, and propose some diagnostic or therapeutic approaches to these mutation-associated cardiac diseases in the future.     

Dual regulation of the cardiac L-type calcium channel in L6 cells by protein kinase C

The role of protein kinase C (PKC) in the regulation of cardiac L-type Ca²⁺ channel activity (LCC) was investigated in L6 rat neonatal myoblasts. Depolarization of fura-2 loaded cells with 140 mM KCl activated a Ba²⁺ influx pathway that was blocked by nifedipine and stimulated by (−) Bay K 8644. At least two splice variants of the α_1C subunit of the cardiac LCC were identified by PCR; the α_1S subunit of the skeletal muscle LCC was not detected. Peptides that specifically inhibit translocation of the novel, Ca²⁺-independent δ and PKC isozymes reduced Ba²⁺ influx by 27% and 19%, respectively, whereas a corresponding peptide directed against translocation of classical PKC α had no effect. Ingenol 3,20-dibenzoate, an agent reported to selectively activate novel PKCs, increased Ba²⁺ uptake by 31% while ethanol, a PKC agonist, enhanced uptake by 38%. In contrast, selective activation of classical PKCs with thymeleatoxin or an agonist peptide reduced Ba²⁺ influx by 23–33%. Ba²⁺ influx was reduced by 30–40% when cells were treated with either a PKC inhibitor (Gö 6983, bisindolylmaleimide) or the PKC activator phorbol-12-myristate-13-acetate. We propose that novel, Ca²⁺-insensitive PKC(s) enhance cardiac Ca²⁺ channel activity in L6 cells under basal conditions while activation of the classical, Ca²⁺-sensitive PKC(s) inhibits channel activity. These findings provide the first evidence that different PKC isozymes exert class-specific opposing effects on cardiac L-type Ca²⁺ channel activity in L6 myoblasts.     

Regulation of the cardiac calcium channel by protein phosphatases

The calcium current (ICa) through the L-type channel in cardiac ventricular cells is enhanced by phosphorylation of a channel protein [Kameyama, M., Hofmann, F. & Trautwein, W. (1985) Pflügers Arch. Eur. J. Physiol. 405, 285-293]. We investigated the possible contribution of the 'catalytic subunits' of protein phosphatase 1 and 2A in the down-regulation of the cardiac calcium channel. Single guinea-pig ventricular myocytes were voltage clamped and the following results were obtained. (1) Intracellular perfusion of the myocyte with the catalytic subunits of protein phosphatase 1 (2 microM) as well as 2A (2.3 microM) completely abolished the increase of ICa induced by isoprenaline (0.05 microM) but did not decrease the basal level of ICa. Alkaline and acid phosphatases were without detectable effect. (2) Cell dialysis with the modulator of protein phosphatase 1 (inhibitor-2) under control conditions (without addition of isoprenaline) caused a slow significant increase of ICa. (3) The time course for the wash-out of the isoprenaline effect was considerably prolonged in the presence of high concentrations of inhibitor-2. (4) Perfusion of the myocyte under basal conditions with adenosine 5'-[gamma-thio]triphosphate led to a slow increase of ICa. Additional superfusion of the cell with a threshold concentration of isoprenaline (0.01 microM) resulted in a rapid increase of ICa which could not be washed out during at least 10 min. From these results we make the following conclusions. (1) The calcium channel from guinea-pig myocytes is regulated by phosphorylation-dephosphorylation. (2) The catalytic subunits of the protein phosphatases 1 as well as 2A, purified from rabbit skeletal muscle, catalyse the down-regulation of the channel. (3) Indirect evidence suggests that endogenous protein phosphatase 1 contributes only partially to the dephosphorylation of the calcium channel in the intact myocyte.     

Photoalteration of calcium channel blockade in the cardiac Purkinje fiber.

Organic compounds that block calcium channel current (calcium antagonists) are important tools for the characterization of this channel. However, the practically irreversible nature of this block restricts the usefulness of this group of drugs. In this paper, we investigate the influence of light on calcium channel blockade by several organic compounds. Our results show that inhibition of calcium channel current by two dihydropyridine derivatives that contain an o-nitro moiety (nisoldipine and nifedipine) can be rapidly reversed by illumination. The energy range important to this reaction is for light wavelengths between 320 and 450 nm. Calcium channel inhibition by two other dihydropyridine derivatives (nicardipine and nitrendipine) as well as by D600, is not modulated by illumination. These results indicate that the photosensitivity of certain dihydropyridine calcium channel blockers make these compounds useful as reversible blockers of this channel.     

The role of region IVS5 of the human cardiac calcium channel in establishing inactivated channel conformation: use-dependent block by benzothiazepines

The role of inactivated channel conformation and use dependence for diltiazem, a specific benzothiazepine calcium channel inhibitor, was studied in chimeric constructs and point mutants created in the IVS5 transmembrane segment of the L-type cardiac calcium channel. All mutations, chimeric or point mutations, were restricted to IVS5, while the YAI-containing segment in IVS6, i.e. the primary interaction site with benzothiazepines, remained intact. Slowed inactivation rate and incomplete steady state inactivation, a behavior of some mutants, were accompanied by a reduced or by a complete loss of use-dependent block by diltiazem. Single channel properties of mutants that lost use dependence toward diltiazem were characterized by drastically elongated mean open times and distinctly slower time constants of open time distribution. Mutation of individual residues of the IVMLF segment in IVS5 did not mimic the complete loss of use dependence as observed for the replacement of the whole stretch. These results establish evidence that amino acids that govern inactivation and the drug-binding site and other amino acids that are located distal from the putative drug-binding site contribute significantly to the function of the benzothiazepine receptor region. The data are consistent with a complex "pocket" conformation that is responsive to a specific class of L-type calcium channel inhibitors. The data allow for a concept that multiple sites within regions of the alpha(1) subunit contribute to auto-regulation of the L-type Ca(2+) channel.     

The bovine cardiac receptor for calcium channel blockers is a 195-kDa protein

The cardiac receptor for calcium channel blockers was purified from bovine microsomal membranes which contained 235 +/- 33 fmol nimodipine-binding sites/mg protein (mean +/- SEM of nine preparations). To identify the receptor during the purification 20% of its binding sites were prelabeled with (+)[3H]PN200-110. The receptor was solubilized with 0.6% digitonin and was purified to a specific density of 157 pmol/mg using a combination of ion-exchange, wheat-germ-agglutinin-Sepharose chromatography and sucrose density gradient centrifugation. In the last sucrose gradient bound (+)[3H]PN200-110 comigrated with a 195-kDa protein. ( +/-)[3H]Azidopine and [3H]ludopamil, the photoaffinity ligands for the dihydropyridine and phenylalkylamine-binding site of the calcium channel, were incorporated specifically into the 195-kDa protein. These data indicate that the bovine cardiac receptor for calcium channel blockers is a 195-kDa protein. Its molecular mass suggests that the bovine cardiac receptor differs considerably from the rabbit skeletal muscle receptor protein for calcium channel blockers.     

Effect of ryanodine on cardiac calcium current and calcium channel gating current.

The effects of 100 microM ryanodine on the L-type calcium channel were studied using the pacth-clamp technique in isolated guinea pig ventricular myocytes. The inactivation kinetics of the calcium current were slowed down in the presence of ryanodine in agreement with the blockade of the release of calcium from the sarcoplasmic reticulum by the drug. The I-V and steady-state inactivation curves of the calcium current were shifted to negative values by ryanodine. A similar shift was observed in the activation and inactivation curves of the intramembrane charge movement associated with the calcium channel. Due to this shift, ryanodine slightly reduced the maximal amount of displaced charge although it did not modify the transition from the inactivated to the activated state (i.e., charge movement repriming). This result is in notable contrast with that obtained in skeletal muscle, where it has been found that ryanodine interferes with charge movement repriming. These results provide additional evidence of the postulated differences between the architecture of the excitation-contraction coupling system in cardiac and skeletal muscle.     

Selectivity and permeation in calcium release channel of cardiac muscle: alkali metal ions

Current was measured from single open channels of the calcium release channel (CRC) of cardiac sarcoplasmic reticulum (over the range +/-180 mV) in pure and mixed solutions (e.g., biionic conditions) of the alkali metal ions Li+, K+, Na+, Rb+, Cs+, ranging in concentration from 25 mM to 2 M. The current-voltage (I-V) relations were analyzed by an extension of the Poisson-Nernst-Planck (PNP) formulation of electrodiffusion, which includes local chemical interaction described by an offset in chemical potential, which likely reflects the difference in dehydration/solvation/rehydration energies in the entry/exit steps of permeation. The theory fits all of the data with few adjustable parameters: the diffusion coefficient of each ion species, the average effective charge distribution on the wall of the pore, and an offset in chemical potential for lithium and sodium ions. In particular, the theory explains the discrepancy between "selectivities" defined by conductance sequence and "selectivities" determined by the permeability ratios (i.e., reversal potentials) in biionic conditions. The extended PNP formulation seems to offer a successful combined treatment of selectivity and permeation. Conductance selectivity in this channel arises mostly from friction: different species of ions have different diffusion coefficients in the channel. Permeability selectivity of an ion is determined by its electrochemical potential gradient and local chemical interaction with the channel. Neither selectivity (in CRC) seems to involve different electrostatic interaction of different ions with the channel protein, even though the ions have widely varying diameters.     

心肌细胞钙信号研究进展

近年自激光共聚焦显微镜使用以来,结合膜片钳技术及分子生物学方法,在心肌细胞内的钙信号种类以及在心脏兴奋－收缩偶联研究方面取得了突破性进展。本文介绍了心肌细胞的钙信号研究进展,包括在心肌细胞内可以观察到的钙闪烁,钙微粒,钙波以及由心肌细胞膜上电除极而诱发的瞬时性钙增高等几种心脏细胞内钙变化的形式,意义以及局部调控兴奋－收给偶联的机制。     

Ion concentration-dependence of rat cardiac unitary L-type calcium channel conductance

Little is known about the native properties of unitary cardiac L-type calcium currents (i(Ca)) measured with physiological calcium (Ca) ion concentration, and their role in excitation-contraction (E-C) coupling. Our goal was to chart the concentration-dependence of unitary conductance (gamma) to physiological Ca concentration and compare it to barium ion (Ba) conductance in the absence of agonists. In isolated, K-depolarized rat myocytes, i(Ca) amplitudes were measured using cell-attached patches with 2 to 70 mM Ca or 2 to 105 mM Ba in the pipette. At 0 mV, 2 mM of Ca produced 0.12 pA, and 2 mM of Ba produced 0.19 pA unitary currents. Unitary conductance was described by a Langmuir isotherm relationship with a maximum gammaCa of 5.3 +/- 0.2 pS (n = 15), and gammaBa of 15 +/- 1 pS (n = 27). The concentration producing half-maximal gamma, Kd(gamma), was not different between Ca (1.7 +/- 0.3 mM) and Ba (1.9 +/- 0.4 mM). We found that quasi-physiological concentrations of Ca produced currents that were as easily resolvable as those obtained with the traditionally used higher concentrations. This study leads to future work on the molecular basis of E-C coupling with a physiological concentration of Ca ions permeating the Ca channel.     

A store-operated calcium channel in Drosophila S2 cells

Using whole-cell recording in Drosophila S2 cells, we characterized a Ca(2+)-selective current that is activated by depletion of intracellular Ca2+ stores. Passive store depletion with a Ca(2+)-free pipette solution containing 12 mM BAPTA activated an inwardly rectifying Ca2+ current with a reversal potential >60 mV. Inward currents developed with a delay and reached a maximum of 20-50 pA at -110 mV. This current doubled in amplitude upon increasing external Ca2+ from 2 to 20 mM and was not affected by substitution of choline for Na+. A pipette solution containing approximately 300 nM free Ca2+ and 10 mM EGTA prevented spontaneous activation, but Ca2+ current activated promptly upon application of ionomycin or thapsigargin, or during dialysis with IP3. Isotonic substitution of 20 mM Ca2+ by test divalent cations revealed a selectivity sequence of Ba2+ > Sr2+ > Ca2+ > Mg2+. Ba2+ and Sr2+ currents inactivated within seconds of exposure to zero-Ca2+ solution at a holding potential of 10 mV. Inactivation of Ba2+ and Sr2+ currents showed recovery during strong hyperpolarizing pulses. Noise analysis provided an estimate of unitary conductance values in 20 mM Ca2+ and Ba2+ of 36 and 420 fS, respectively. Upon removal of all external divalent ions, a transient monovalent current exhibited strong selectivity for Na+ over Cs+. The Ca2+ current was completely and reversibly blocked by Gd3+, with an IC50 value of approximately 50 nM, and was also blocked by 20 microM SKF 96365 and by 20 microM 2-APB. At concentrations between 5 and 14 microM, application of 2-APB increased the magnitude of Ca2+ currents. We conclude that S2 cells express store-operated Ca2+ channels with many of the same biophysical characteristics as CRAC channels in mammalian cells.     

Dihydropyridines as potent calcium channel blockers in neuronal cells

Nicardipine, one of the dihydropyridine derivatives, in a nanomolar concentration range suppressed the high K+ -induced neurotransmitter release from cultured neuronal cells (chick embryonic neural retina cells and clonal rat pheochromocytoma cells). The high K+ -induced Ca2+ uptake into pheochromocytoma cell was also blocked by nicardipine in the same concentration range. [3H]Nitrendipine, another dihydropyridine derivative, bound specifically to pheochromocytoma cell homogenate in a saturable manner. We concluded that dihydropyridines block and bind to the high K+ -sensitive Ca2+ channels in neuronal cells.     

Diffusion around a cardiac calcium channel and the role of surface bound calcium.

The diffusion of Ca as it converges to the external mouth of a Ca channel is examined. Diffusional limitation on Ca ions entering Ca channels during current flow, cause local extracellular Ca depletions. Such extracellular Ca depletions have been reported in cardiac muscle. The cardiac sarcolemma has a large number of low-affinity Ca binding sites that can buffer these local Ca depletions. For a hemisphere of extracellular space (of radius less than 0.33 microns) centered on the external mouth of a Ca channel the amount of Ca bound at the membrane surface exceeds that which is free within the associated hemisphere. The ratio of bound Ca/free Ca increases as r decreases, such that the [Ca] nearest the Ca channel is the most strongly buffered by sarcolemmal bound Ca. It is demonstrated that Ca ions coming from these sarcolemmal Ca binding sites contribute quantitatively to the integrated Ca current. The electric field generated by the local depletion of Ca near the channel mouth has little impact on the extent of Ca depletion, but if an additional electric field exists at the mouth of the channel, Ca depletion can be significantly altered. Other low-affinity Ca binding sites in the interstitium may also contribute to the buffering of extracellular Ca. The complex geometry of the extracellular space in cardiac muscle (e.g., transverse tubules and restrictions of extracellular space between cells) increases both the predicted Ca depletions (in the absence of binding) and the bound/free ratio. Thus, the impact of this surface Ca binding is greatly increased. By considering arrays of Ca channels in transverse tubules or in parallel planes (e.g., membranes of neighboring cells), extracellular Ca depletions are predicted which agree with those measured experimentally. Membrane Ca binding may also be expected to buffer increases in [Ca] around the inner mouth of Ca channels. It is demonstrated that in the absence of other intracellular systems most of the Ca entering the cell via Ca channels might be expected to be bound to the inner sarcolemmal surface. It is concluded that surface Ca binding may have a substantial impact on the processes of extracellular Ca depletion (and intracellular Ca accumulation).     

Numerical simulation of local calcium movements during L-type calcium channel gating in the cardiac diad.

Computer simulation was used to investigate the calcium levels after sarcolemmal calcium influx through L-type calcium channels (DHPRs) into the narrow diadic space of cardiac muscle. The effect of various cytosolic and membranebound buffers, diad geometry, DHPR properties (open time and current), and surface charge were examined. The simulations showed that phospholipid binding sites on the sarcolemmal membrane are the major buffer affecting free calcium ([Ca2+]) levels in the diad. The inclusion of surface charge effects calculated from Gouy-Chapman theory resulted in a marked decrease in [Ca2+] levels at all times and a faster decay of [Ca2+] after termination of DHPR influx. For a DHPR current of 200 fA, [Ca2+] at the center of the diad reached peak levels of approximately 73 microM. In larger diads (> or = 400 nm diameter), [Ca2+] decayed more slowly than in smaller diads (100-200 nm diameter), although peak [Ca2+] levels reached during typical DHPR open times were similar. For a wide range of DHPR single-channel current magnitudes (Ica = 25-200 fA), [Ca2+] levels in the diad were approximately proportional to ICa. The decrease in calculated [Ca2+] levels due to the effects of surface charge can be interpreted as resulting from an effective "volume expansion" of the diad space. Furthermore, the layer of increased [Ca2+] close to the sarcolemmal membrane can act as a fast buffer.